Probably too simple to be useful. Doesn't seem to support overlapping times. Uses "[br]" for line breaks. Unclear how easy it would be to extend usefully without effectively making a completely new format.

SubStation Alpha (.ssa)

Pretty verbose. Has some pixel-based positioning. Uses "\N" for line breaks (though this is somewhat configurable). Has some rather subtle syntax of the kind that typically results in buggy implementations. (Indeed, see the "note" in [1].)

A later version called Advanced SubStation Alpha (.ass) is even more complicated.

Either uses "[br]" for line breaks, "|" for line breaks, or real line breaks, depending on the version. Making a backwards-compatible parser would likely require more effort than for most other formats.